For the purposes of discussion it will be assumed that the term "rotating shaft" or just "shaft" includes wear sleeves providing the actual rotating surface in contact with an oil seal, as well as those shafts with which the oil seal makes actual contact.
In spite of the many years and great efforts that have been spent on the arts relating to lubrication, perfect understanding is still unattained. The present invention deals with the relationship between a rotating shaft, a radial-lip type of oil seal, and a housing. The retention of the oil seal in the housing and the assurance of leak-tightness there is not pertinent to the present invention and will be assumed to be present. It is readily obtainable since there is no movement between the installed oil seal case and the housing.
For the retention of oil inside a housing for a rotating shaft, it is common practice to rely on a radial-lip type of oil seal having a casing held in a leak-tight fit with the housing. Such an oil seal has an elastomeric lip for leak-tight engagement with the surface of a relatively rotating shaft. At least three elements are necessary for the retention of oil: (1) the elastomeric ring or shaft seal, (2) the shaft, and (3) the lubricating oil or other fluid which is to be sealed. The lubricating oil provides a very narrow film separating the elastomer from the rotating shaft and thereby preventing the destruction of the elastomer.
Heretofore, it has generally been assumed that a shaft or wear sleeve which has been ground and polished provides an absolutely smooth surface which has nothing to do with the problem of lubricating oil retention unless it is marred by defects such as scratches which are quite visible to the naked eye. It has also been assumed that a shaft which feels smooth and looks smooth is smooth, and that any problem that may arise leading to loss of some of the oil is therefore due to and the fault of the elastomeric sealing element.
The present invention is based upon the discovery that the assumption that a smooth-appearing and smooth-feeling shaft plays no part in the possible leakage of oil is not strictly true. The grinding of the shaft to provide the "smooth" cylindrical surface that is necessary, produces microscopically observable irregularities which have heretofore been largely neglected but which, it has been discovered, can result in oil leakage.
The problem has been that, during grinding, by either the conventional technique of plunge grinding or centerless grinding, the longitudinal axis of both the shaft being ground and the grinder, while they have been assumed to be parallel, have in fact been askew, due to tolerances generally permitted such parallelism. Until only recently (as mentioned below relative to an earlier patent application having Ser. No. 741,533 filed by co-workers on Apr. 26, 1985) the importance of eliminating all such tolerances was not recognized.
This askewness during grinding creates a directionally biased series of non-smooth microabrasions on the shaft, resulting in something analogous to a screw thread, and imparting a liquid-pumping effect with similarities to the effect of an Archimedes' screw. It will be recalled that this screw, whether invented by Archimedes or by others before his time, comprises a rather broad threaded screw encased by a cylinder and was long used in Egypt and elsewhere to raise water. In the present case, the cylinder may be considered as analogous to the radial lip shaft seal, while the screw is provided by the shaft turning within the seal, and the liquid is typically oil. Thus, depending on the direction of shaft rotation, the oil may be pumped away from the lip, or the oil may be pumped in the opposite direction so that the oil flows beyond the seal, producing leakage. In the latter case, the user has usually believed that he has a faulty seal, and he may have changed the seal, only to discover that the problem remained without anybody knowing why.
The ignorance of this problem is such that neither the ultimate users of shafts, nor the shaft manufacturers, nor the oil seal manufacturers have heretofore appreciated it. As a result, when new designs or new types of oil seals were tested, leakage sometimes occurred for no apparent reason that could be determined. On retrospect it appears that the reason was this Archimedes' screw effect due to the directional effect of the grinding of the shaft--even though the shaft seemed to be perfectly smooth and met all the established surface smoothness specifications.
The grinding of a shaft, just discussed, does not result in what is actually a screw thread, but it is similar in that it creates microscopic troughs, all extending in the same general direction, namely, that governed by the degree of askewness, and each individually and collectively tending to give the effect of a screw thread pattern. The angle is so small that it cannot be seen even with a 250.times. power microscope, but its existence can be verified by a variety of tests. On the other hand, many instances of dynamic oil leakage, i.e., during relative rotation with a shaft, can actually be traced to this basic phenomenon.
A typical example of a shaft intended for use in the automotive or light truck vehicular market is given below in the detailed description of my invention. However, suffice it to say that literally hundreds, if not thousands, of these microscopic troughs on the shaft surface are within the seal lip contact area with the shaft and each imparts an axial force on the thin film of lubricant, commonly oil, generally present between the seal and shaft within this limited area. As a result, the lubricant is literally pumped in one axial direction or the other dependent on relative direction of shaft rotation out from the seal-shaft contact area. In one such direction of shaft rotation, the seal will be considered a "leaker". Although the effect is not gross, the effect over a long period of time, and in some particular environments, will be serious enough that the user of the device will often believe that his oil seal is leaking and will replace it under the faulty assumption that the oil seal is defective or worn out, when that is not the case at all.
The dynamic oil leakage problem may or may not be severe, but the effect on the user, as already noted, may be severe because no one wants to see oil leaking in any of the situations in which oil seals are used, and if the wrong remedy is attempted, considerable expense can result without any beneficial result being obtained.
An earlier patent application having Ser. No. 741,533 conceived by co-workers and subject to the same ownership as this application has been filed on Apr. 26, 1985, which clearly recognizes the problem as being caused by askewness between grinder and shaft, as aforesaid, and proposes as a solution to either (i) eliminate virtually all tolerance heretofore permitted in establishing parallelism between grinder and shaft so as to produce a no-lead shaft or (ii) grind the shaft at a known degree of non-parallelism with the grinder so that the direction of shaft lead could be accurately controlled during production, identified, and taken advantage of during installation, to provide a shaft seal combination which would pump the thin oil film in the direction of the oil reservoir rather than to the air side of the seal. This, however, does not solve the problem that occurs when the shaft is intended to rotate in both directions, such as a transmission drive or drive axle, or where for production reason a compatible direction of shaft rotation can not be assured, and it is to this specific problem that the present invention is directed.